The present invention refers to a furnace comprising a fire tube, and a final combustion chamber located axially downstream of the fire tube, and in which a burner is mounted at the end of the fire tube remote from said chamber and a restricted passage is formed between the fire tube and the final combustion chamber, said passage being formed in a partition wall defining the inward end of the fire tube, the surface of said wall, presented towards the fire tube sloping towards said passage.
Such furnaces have fine combustion properties. The aim of the present invention is to simplify the design, and to further improve the combustion.